Why do people write out extra filler words? It's bad enough people vocalize the "like", "umm", "or whatever", "sorta", "ok, so...", "well...", etc, but why do they even go to the trouble and effort of typing out these worthless adders?

It's a habit for most people. We tend to type the way we speak.

I would type ummm as a sarcastic word or hmmm to show that I was thinking

I do it because it simulates actual conversation and helps me express or emphasize the way I feel or what I'm trying to say.

It's a habit, doesn't it make sense that someone would type how they talk?

If you are not very knowledgeable about writing, consider that an "um. . ." or an "er. . ." could express sarcasm, confusion, humor, or even a diminishing of the importance of the question or questioner. Further, people who use these are often more fully expressing themselves than those who don't. So I would have to say that I disagree with you.
People who use shortened form of words (u, tkn, etc.) and those who use initials for none-standard abbreviations (TH for Tokio Hotel, for example) ARE annoying.

To add interest, personality and tone to a written work. In the text medium much is lost in communication such as body language, inflection and vocal tone, these additions can allow subtle or not so subtle indications of the writer's intent and mood and can make, for some, a more interesting read. Overused, however, it can be annoying. These additions can also give writers identity and allows them to be recognized by their style.

Basic communication skills to give "life" to dry text.

J.